High frequency conductors can be laminated, and therefore can be electrically anisotropic, featuring low resistance in the direction of the current flow and high resistance in the direction normal to the current flow, which in turns results in low RF losses. At high frequencies, the AC resistance of a conductor is related to the skin depth of the conductor, which is dependent on the permeability of the conductor material, the operating frequency, and the DC resistivity of the conductor material. As the frequency increases, the skin depth becomes smaller, which means that the resistance of a conductor of a given thickness increases with frequency. The loss associated with this phenomenon is due to eddy currents formed within the conductor. In spiral RF inductors, the inductor quality factor is limited by the conductor loss (a high quality factor yields higher circuit efficiency). Reducing RF conductor losses is a key challenge when manufacturing spiral RF inductors. This problem can be addressed by laminating the conductor material.
The document entitled: “Reduction of Skin-Effect Losses by the Use of Laminated Conductors”, by A. M. Clogston, Bell Labs, Proceedings of the IRE (Volume: 39, Issue: 7), July 1951, p 767-782, discusses the reduction of skin-effect losses in transmission lines by laminated conductors. The theory is presented for both infinitesimally thin laminated layers and laminated layers of finite thickness.
The document entitled: “Reduction in Ohmic Loss of Small Microstrip Antennas using Multiple Copper Layers”, by Saeed I. Latif et al., Antennas and Propagation Society International Symposium 2006, IEEE, pp 1625-1628 demonstrates analytically that multiple laminated conducting material layers reduce the ohmic losses when compared against a single solid layer.
U.S. Pat. No. 2,831,172, entitled: “A Laminated conductor”, to Bell Telephone Labor Inc, April 1958, discloses composite conductors formed of a multiplicity of insulated conducting portions which have come to be known as “Clogston conductors”.
U.S. Pat. No. 6,148,221, entitled: “A Thin film multilayered electrode of high frequency electromagnetic field coupling”, to Murata Manufacturing, November 2000, discloses a plurality of TEM mode transmission lines (L2-L5) structured by pairs of thin film conductors (21 and 22, 22 and 23, 23 and 24, and 24 and 25) which sandwich thin film dielectrics (31 to 34) by alternately stacking the thin film conductor (21 to 25) and the thin film dielectric (31 to 34). The phase velocities of TEM mode waves which are propagated at least by two of the transmission lines (L2 to L5) are substantially equal to each other. The thickness of each of the thin film conductors (21 to 25) is smaller than the skin depth of the frequency used so that the electromagnetic fields of at least two TEM mode transmission lines among the TEM mode transmission lines (L2 to L5) are coupled to each other. In this way, the skin depth can be increased effectively. The conductor loss and the surface resistance can be reduced significantly as compared to those of the conventional electrode. By use of this electrode, a transmission line, a resonator, a filter, and a high frequency device are structured.
Published U.S. Patent application No. 20140376199, entitled: “Laminated multi-conductor cable”, to Noboru Kato, 2014, describes a laminate body that includes a plurality of dielectric sheets laminated together. A first ground conductor is provided in or on the laminate body. A second ground conductor is provided in or on the laminate body and located on a different layer from the first ground conductor. A signal line is provided between the ground conductors and with respect to a direction of lamination. A signal line is provided between the ground conductors and with respect to the direction of lamination and located closer to the second ground conductor than the signal line is, and the signal line has a portion extending along the signal line in a parallel-lines area when viewed from the direction of lamination. The first ground conductor has openings in the parallel-lines area, and the openings are arranged over the signal line when viewed from the direction of lamination.
Published U.S. Patent application No. US 20040164839 A1, entitled: “Magnetic inductor core and inductor and methods for manufacturing same”, to Georgia Tech Research Corporation, describes a highly-laminated magnetic inductor core and integrated inductor, and methods for making the same. A representative method for manufacturing a highly-laminated magnetic inductor core includes: depositing at least a first layer of a ferromagnetic material; depositing at least a first layer of a sacrificial conductive material; depositing a support structure formed of a ferromagnetic material; and removing the sacrificial conductive material, thereby leaving the at least first layer of ferromagnetic material mechanically supported by the support structure.
Published U.S. Patent application No. US 20130062729 A1, entitled: “Forming a ferromagnetic alloy core for high frequency micro fabricated inductors and transformers”, to Texas Instruments, describes a plurality of sequential electro-deposition, planarization and insulator deposition steps performed over a patterned thick photoresist film to form a laminated ferromagnetic alloy core for micro-fabricated inductors and transformers. The use of a plurality of contiguous thin laminations within deep patterns on non-removable photoresist film provides sufficient volume of magnetic film in, for example, high frequency applications, and reduces eddy current loss at high frequency.
U.S. Pat. No. 4,614,563 A, entitled “Process for producing multilayer conductor structure, Fuji Photo Film Co., September 1986” describes a process for producing a multilayer conductor structure having at least two conductor patterns in layers and a smooth surface over the upper pattern, which comprises the steps of: (1) forming a first conductor pattern on a substrate or a layer provided on the substrate; (2) providing a conductor layer over an intermediate insulation layer formed on said first conductor pattern; (3) providing a resin layer on said conductor layer to form a smooth surface thereover; and (4) etching said resin layer and a part of said conductor layer provided on the first conductor pattern to form a second conductor pattern having a smooth surface thereon.
PCT published patent application WO2014121100, entitled “Multilayer conductors with integrated capacitors and associated systems and methods” to THE TRUSTEES OF DARTMOUTH COLLEGE, 2014, discloses a multilayer conductor that includes at least one separation dielectric layer and a plurality of conductor layers stacked in an alternating manner. Each of the plurality of conductor layers includes a first conductor sublayer and a second conductor sublayer separated from the first conductor sublayer by a sublayer dielectric layer. The second conductor sublayer at least partially overlaps with the first conductor sublayer in each of the plurality of conductor layers. The multilayer conductor is included, for example, in a device including a magnetic core adjacent to at least part of the multilayer conductor.